Thermoplastic elastomers having enhanced foaming and physical properties

ABSTRACT

An olefinic thermoplastic elastomer composition which includes an acrylic-modified polytetrafluoroethylene. When foamed, such compositions produce a very soft foam with improved processing properties and physical characteristics. A process of foaming and foamed articles are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to thermoplastic elastomer (TPE)materials. Thermoplastic elastomers are broadly defined as rubber-likematerials that, unlike conventional vulcanized rubbers, can be processedand recycled like thermoplastic materials, yet have properties andperformance similar to that of vulcanized rubber at servicetemperatures. The invention more specifically relates to thermoplasticvulcanizates (TPV), which are thermoplastic elastomers with across-linked rubbery phase produced by the process of dynamicvulcanization. The thermoplastic vulcanizates of the invention arefoamed materials produced by physical or chemical blowing agents,wherein processing characteristics and foam properties are improved bythe inclusion of a modifier derived from polytetrafluoroethylene. Theinvention also relates to foamed articles obtainable by the process ofthe invention.

2. Description of the Prior Art

There has been considerable activity on the development of thermoplasticvulcanizate compositions, especially those based on polyolefinthermoplastic resins, which have good foaming properties, and onprocesses for producing foams having improved properties. U.S. Pat. No.5,070,111, incorporated herein by reference, discloses a process offoaming thermoplastic elastomer compositions using water as the solefoaming agent. U.S. Pat. Nos. 5,607,629 and 5,788,889, both incorporatedherein by reference, describe methods for the production of foamedthermoplastic elastomer profiles by extrusion with a water blowingagent. U.S. Pat. No. 5,824,400 discloses foamed thermoplastic elastomercompositions which incorporate styrenic elastomers. Published EuropeanPatent Application No. 0 860 465 teaches a method of foamingthermoplastic elastomers using a water containing chemical compoundwhich releases water at temperatures above the melting point of thethermoplastic elastomer. Published European Patent Application 0 872 516discloses the use of polypropylene resins having specific rheologicalproperties to enhance the foaming performance of olefinic thermoplasticelastomers.

However, the problems of providing thermoplastic elastomer foams whichare soft, with good surface smoothness, low water absorption, improvedcompression set and compression load deflection, and having fine anduniform cell structure have not been overcome by prior art.

SUMMARY OF THE INVENTION

The present invention is based on the discovery that superiorthermoplastic elastomer foams can be produced by incorporating into thethermoplastic elastomer, prior to foaming, an acrylic-modifiedpolytetrafluoroethylene. Incorporation of this additive provides a verysoft foam product having a number of desirable attributes, includingimproved processability, high melt strength, high cell density anduniformity, smooth surface, low water absorption, with improvedcompression set and compression load deflection.

In detail the present invention relates to a process for foaming athermoplastic elastomer using a physical or chemical blowing agent,wherein an acrylic-modified polytetrafluoroethylene is incorporated intothe thermoplastic elastomer composition prior to foaming. Sufficientacrylic-modified polytetrafluoroethylene is incorporated to be effectivein achieving the desired attributes. The invention also encompassesthermoplastic elastomer compositions containing the acrylic-modifiedpolytetrafluoroethylene, and foamed articles prepared therefrom.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Thermoplastic Elastomer

Thermoplastic Resin Component

Thermoplastic resins suitable for use in the compositions of theinvention include thermoplastic, crystalline polyolefin homopolymers andcopolymers. They are desirably prepared from monoolefin monomers having2 to 7 carbon atoms, such as ethylene, propylene, 1-butene, isobutylene,1-pentene, 1-hexene, 1-octene, 3-methyl-1-pentene, 4-methyl-1-pentene,5-methyl-1-hexene, mixtures thereof and copolymers thereof with(meth)acrylates and/or vinyl acetates. Preferred, however, are monomershaving 3 to 6 carbon atoms, with propylene being most preferred. As usedin the specification and claims the term polypropylene includeshomopolymers of propylene as well as reactor and/or random copolymers ofpropylene which can contain about 1 to about 30 weight percent ofethylene and/or an alpha-olefin comonomer of 4 to 16 carbon atoms, andmixtures thereof. The polypropylene can have different types ofmolecular structure such as isotactic or syndiotactic, and differentdegrees of crystallinity including materials with a high percentage ofamorphous structure such as the “elastic” polypropylenes. Furtherpolyolefins which can be used in the invention are high, low, linear-lowand very low density polyethylenes, and copolymers of ethylene with(meth)acrylates and/or vinyl acetates.

The polyolefins mentioned above can be made using conventionalZiegler/Natta catalyst systems or by single site catalyst systems.Commercially available polyolefins may be used in the practice of theinvention.

The amount of thermoplastic polyolefin resin found to provide usefulthermoplastic elastomer compositions is generally from about 8 to about90 weight percent. Preferably, the thermoplastic polyolefin content willrange from about 9 to about 60 percent by weight.

Elastomer Component

Suitable rubbers include non-polar, rubbery copolymers of two or morealpha-monoolefins, preferably copolymerized with at least one polyene,usually a diene. Saturated monoolefin copolymer rubber, for exampleethylene-propylene copolymer rubber (EPM) can be used. However,unsaturated monoolefin rubber such as EPDM rubber is more suitable. EPDMis a terpolymer of ethylene, propylene and a non-conjugated diene.Satisfactory non-conjugated dienes include 5-ethylidene-2-norbornene(ENB); 1,4-hexadiene; 5-methylene-2-norbornene (MNB); 1,6-octadiene;5-methyl-1,4-hexadiene; 3,7-dimethyl-1,6-octadiene; 1,3-cyclopentadiene;1,4-cyclohexadiene; dicyclopentadiene (DCPD); and vinyl norbornene(VNB).

Butyl rubbers are also useful in the thermoplastic elastomercompositions. As used in the specification and claims, the term butylrubber includes copolymers of an isoolefin and a conjugated monoolefin,terpolymers of an isoolefin with or without a conjugated monoolefin,divinyl aromatic monomers and the halogenated derivatives of suchcopolymers and terpolymers. Another suitable copolymer within the scopeof the olefin rubber of the present invention is a copolymer of a C₄₋₇isomonoolefin and a para-alkylstyrene, and preferably a halogenatedderivative thereof. The amount of halogen in the copolymer,predominantly in the para-alkylstyrene, is from about 0.1 to about 10weight percent. A preferred example is the brominated copolymer ofisobutylene and para-methylstyrene. Natural rubbers are also olefinrubbers suitable for use in the thermoplastic elastomer composition.

The amount of rubber in the thermoplastic elastomer generally rangesfrom about 92 to about 10 weight percent. Preferably the olefin rubbercontent will be in the range of from about 40 to about 91 weightpercent.

Additives

The thermoplastic elastomer may optionally contain reinforcing andnon-reinforcing fillers, plasticizers, antioxidants, stabilizers, rubberprocessing oils, extender oils, lubricants, antiblocking agents,antistatic agents, waxes, foaming agents, pigments, flame retardants andother processing aids known in the rubber compounding art. Suchadditives may comprise up to about 70 weight percent, more preferably upto about 65 weight percent, of the total composition. Fillers andextenders which can be utilized include conventional inorganics such ascalcium carbonate, clays, silica, talc, titanium dioxide, carbon blackand the like. The rubber processing oils generally are paraffinic,napthenic or aromatic oils derived from petroleum fractions. The oilsare selected from those ordinarily used in conjunction with the specificrubber or rubber component present in the composition.

In one particularly preferred embodiment of the invention, the inclusionof an adsorptive inorganic additive has been found to improve the odorproperties of the foamed products. The addition of an additive such asmagnesium oxide in the range of about 0.1 to about 3 weight percent,preferably about 0.5 to about 2 weight percent, based on the totalcomposition, is effective in eliminating odors.

Processing

The rubber component of the thermoplastic elastomer is generally presentas small, i.e. micro size, particles within a continuous thermoplasticresin matrix, although a co-continuous morphology or a phase inversionis also possible depending upon the amount of rubber relative tothermoplastic resin and the degree of vulcanization, if any, of therubber. Preferably, the rubber is at least partially vulcanized, andmost preferably it is fully vulcanized (crosslinked).

The partial or full crosslinking can be achieved by adding anappropriate rubber curative to the blend of thermoplastic olefin polymerand olefin rubber, and vulcanizing the rubber to the desired degreeunder vulcanizing conditions. It is preferred that the rubber becrosslinked by the process of dynamic vulcanization. As used in thespecification and claims, the term dynamic vulcanization means avulcanization or crosslinking (curing) process wherein the rubber isvulcanized under conditions of shear at a temperature above the meltingpoint of the polyolefin component.

Those of ordinary skill in the art will appreciate the appropriatequantities and types of vulcanizing agents, and the conditions requiredto achieve the desired vulcanization. Any known crosslinking system canbe used, so long as it is suitable under the vulcanization conditionsfor the elastomer component and it is compatible with the thermoplasticolefin polymer component of the composition. Crosslinking (curing)agents include sulfur, sulfur donors, metal oxides, phenolic resinsystems, maleimides, peroxide based systems, hydrosilylation systems,high energy radiation and the like, both with and without acceleratorsand co-agents.

The terms fully vulcanized or completely vulcanized as used herein meanthat the olefin rubber component of the composition has been crosslinkedto a state in which the elastomeric properties of the crosslinked rubberare similar to those of the rubber in its conventional vulcanized state,apart from the thermoplastic elastomer composition. The degree ofcrosslinking (or cure) of the rubber can also be expressed in terms ofgel content, crosslink density or amount of uncrosslinked rubber whichis extractable by a rubber solvent. All of these descriptions are wellknown in the art. A typical partially crosslinked composition will haveless than about 50 to less than about 15 weight percent of the elastomerextractable by a rubber solvent, while a fully crosslinked compositionwill have less than about 5 weight percent, and preferably less thanabout 3 weight percent, of the elastomer extractable by a rubbersolvent.

Usually about 5 to about 20 parts by weight of the crosslinking agent orsystem are used per 100 parts by weight of the rubber component to bevulcanized.

As used herein, the terms thermoplastic elastomer and thermoplasticvulcanizate refer to blends of polyolefinic thermoplastic resin andvulcanized [cured; cross-linked] rubber [elastomer]. Such materials havethe characteristic of elasticity, i.e. they are capable of recoveringfrom large deformations quickly and forcibly. One measure of thisrubbery behavior is that the material will retract to less than 1.5times its original length within one minute, after being stretched atroom temperature to twice its original length and held for one minutebefore release (ASTM D1566). Another measure is found in ASTM D412, forthe determination of tensile set. The materials are also characterizedby high elastic recovery, which refers to the proportion of recoveryafter deformation and may be quantified as percent recovery aftercompression. A perfectly elastic material has a recovery of 100% while aperfectly plastic material has no elastic recovery. Yet another measureis found in ASTM D395, for the determination of compression set.

Modified Polytetrafluoroethylene

The composition of the invention includes an acrylic-modifiedpolytetrafluoroethylene (PTFE) component. This component is generallydescribed as a mixture of a polytetrafluoroethylene and alkyl(meth)acrylate having from 5 to 30 carbon atoms. One such blend which isparticularly suited for use in the process of the invention is availableas Metablen™ A-3000, available from Mitsubishi Rayon Co., Ltd.

The amount of the modified polytetrafluoroethylene component in thecomposition of the invention generally ranges from about 0.1 to about 4weight percent, based on the total weight of the composition includingthe thermoplastic resin component, the rubber component, additives andthe modified polytetrafluoroethylene component. The preferred amount ofmodified polytetrafluoroethylene ranges from about 0.5 to about 2 weightpercent, with about 1 to about 2 weight percent being most preferred.Alternatively, the amount of acrylic-modified polytetrafluoroethylenecan be expressed in terms of the total weight of thermoplastic resin andmodified polytetrafluoroethylene. The preferred amount of modifiedpolytetrafluoroethylene, expressed in this manner, ranges from about 8to about 30 weight percent with a range of about 15 to about 30 weightpercent being most preferred.

In the preparation of thermoplastic elastomers of the invention, theacrylic-modified polytetrafluoroethylene was generally incorporateddirectly into the thermoplastic elastomer during production of thethermoplastic elastomer so that it was an integral part of thecomposition. Alternatively, the acrylic-modified polytetrafluoroethylenecan be mechanically blended with a preformed thermoplastic elastomercomposition, or it can be introduced into the foaming processsimultaneously with the thermoplastic elastomer.

EXAMPLES

The combined thermoplastic elastomer and acrylic-modifiedpolytetrafluoroethylene was fed into an extruder or other mixing devicecapable of maintaining melt temperatures in the range of about 165° C.to about 220° C. If the blowing agent was a solid material, it was alsoblended with the thermoplastic elastomer prior to introduction into themixing device. When the blowing agent was a gas or liquid, it wasinjected into the mixing device through an appropriate inlet. Theblowing agent was thus thoroughly dispersed in the molten thermoplasticelastomer, and the mixture was maintained at a pressure sufficient toprevent premature foaming. The mixture was passed through a die or otherappropriate outlet, where foaming occurred. The foamed product wascooled in air or in a water mist.

In the following examples thermoplastic elastomers were prepared fromblends of polypropylene thermoplastic resin and EPDM rubber, with commonadditives and processing aids. Acrylic-modified polytetrafluoroethylenewas incorporated into the blends and the rubber component wascross-linked by dynamic vulcanization using a phenolic resin curesystem. For the fabrication of foamed articles by an extrusion process,the thermoplastic elastomer was introduced into a single screw extruderand thoroughly melted. The blowing agent, water in the examples setforth in Table 1, was then injected under pressure into the moltenthermoplastic elastomer at rates of 1.1 to 1.4 weight percent. The meltwas mixed and conveyed, under pressure, to the extruder exit and througha shaping die. The hot and fragile foam was transferred to a conveyorbelt where it was cooled by air and water mist. The foamed article maythen be cut or shaped for specific applications. Foamed profiles can beeither extruded alone as described or coextruded with a dense carrier.

The following measurement methods were used in evaluating the examplesof the invention:

Tensile strength at break; tensile set; tensile modulus; elongation atbreak—ASTM D412 (ISO 37, type 2)

Shore hardness—ASTM D2240

Specific gravity—ASTM D792

Surface (Ra)—Surface finish was evaluated as the arithmetic average ofroughness irregularities measured from a mean line with the samplinglength, using a Surface Analyzer System from Federal ProductsCorporation, Providence, R.I.

Compression set—The sample was compressed inside spaced sample holdersto 40% of its initial height, and held at 100° C. for 22 hours. Thesample was removed and allowed to recover for 30 minutes at roomtemperature. Compression set was then determined as:CS(%)=(H_(initial)−H_(final))/(H_(initial)−H₀)×100, where H₀ is the gapof the sample holder (60% of H_(initial)).

Compression load deflection—The force necessary to compress a 100 mmsample to 40% of its original height, at room temperature.

Water absorption—Two test methods were used to measure water absorption.In the first method (A) a weighed foam profile 50 mm long was submergedin water at room temperature two inches below the surface of the water.The specimen was allowed to remain submerged for either 24 hours atatmospheric pressure, or for three minutes at 23 inches Hg vacuum (abovethe surface of the water). After the appropriate time, the specimen wasremoved, blotted dry, weighed and the percent change in mass wascalculated. In the second method (B) a weighed foam profile 254 mm longwas submerged in water at room temperature eight inches below thesurface of the water, with a one inch section of the specimen locatedabove the water at each end. The specimen was allowed to remain thussubmerged for either 24 hours at atmospheric pressure, or for fiveminutes at 26 inches Hg vacuum (above the surface of the water). Afterthe appropriate time, the specimen was removed, dried, weighed and thepercent change in mass was calculated. TABLE 1 Con. Con. Example

A B 1 2 3 4 Components

(parts by weight) Polypropylene 42 32 32 32 28 32 EPDM rubber 100 100100 100 100 100 Process oil 150 150 150 150 150 150 Additives/Curative63 63 63 63 63 63 Metablen ™ A3000 0 0 3 7 7 7 TPE Properties

Hardness (Shore A) 66 62 61 61 54 57 Ultimate tensile strength 6.9 5.35.2 4.7 4.2 4.4 (MPa) Modulus - 100 (MPa) 2.71 1.94 2 1.7 1.5 1.63Ultimate elongation (%) 520 368 330 317 389 382 Tensile set (%) 8 9 108.5 9 Viscosity (poise) 353 544 603 672 707 729 Foam Properties

Specific gravity (1.1 wt % 0.47 0.45 0.47 0.51 0.55 water) Specificgravity (1.4 wt % 0.45 0.4 0.4 0.42 0.45 0.49 water) Surface (microns)9.1 8.1 7.8 6.3 8 7.4 H₂O absorption (test A) - 38 6.1 2.4 4.8 4.8 2.8Atmosphere (%) H₂O absorption (test A) - 50 16.4 14.4 2.8 4.5 3.4 Vacuum(%) Compression set (%) 52 30 33 31 44 44 Compression load 0.77 0.6 0.510.56 0.31 0.5 deflection (kg)

As can be seen from the examples, the foamed thermoplastic elastomer ofthe invention provides a smooth surface, low water absorption, goodcompression set and improved compression load deflection. Visualinspection shows that foam cell density is high and the cells areuniform in structure with cell size distribution in a narrow range.Microscopy indicates that about 60% of the cells have a diameter of lessthan 100 microns.

Additional examples were prepared using different thermoplasticelastomer formulations, and foams were generated using various levels ofwater as the blowing agent. The foam properties were evaluated, and theresults are set forth in Tables 2 and 3. TABLE 2 Example

5 6 7 Components

(weight %) EPDM rubber 49.7 49.3 49 Polypropylene 9.1 9 9 Process oil21.3 21.1 21 Clay 11.9 11.8 11.8 ZnO 0.6 0.6 0.6 SnCl₂ 0.4 0.3 0.3Curative 1.1 1.1 1.1 Carbon black 4 3.9 3.9 Modified PTFE 2 2 2 MgO 00.8 1.4 TPE Properties

Hardness (Shore A) 60 60 60 Ultimate tensile strength (MPa) 6.3 5.1 3.9Modulus - 100 (MPa) 2 1.8 1.7 Ultimate elongation (%) 390 470 510Viscosity (poise) 375 685 485

TABLE 3 Example

5 6 7 Blowing agent (wt %) 0.9 1.3 1.8 0.9 1.3 1.8 0.9 1.3 1.8 Density0.59 0.47 0.42 0.55 0.43 0.34 0.6 0.46 0.37 Surface (microns) 6.6 8.111.6 5.7 5.2 5.6 5.1 5.4 6.5 H₂O absorption (test 0.8 1.7 1.8 0.5 0.70.5 0.3 0.5 0.8 B) - Atmosphere (%) H₂O absorption (test 0.15 0.16 0.180.5 0.1 0.1 1 1.5 2.7 B) - Vacuum (%) Compression set (%) 31 27 27 41 4240 47 55 63 Compression load 2.1 1.9 1.8 1 0.8 0.6 1 0.6 0.6 deflection(%)

The materials used in the thermoplastic elastomers of Table 2 were EPDMrubber—Vistalon™ 3666 (ExxonMobil Chemical Co.); polypropylene—D008M™(Aristech Chemical Corp.); process oil—Sunpar™ 150M; clay—Icecap K™(Burgess); curative—SP-1045™ (Schenectady International); carbonblack—Ampacet 49974 (Ampacet Corp.); modified PTFE—Metablen™ A3000(Mitsubishi Rayon Co., Ltd.).

The foamed thermoplastic elastomer composition and the molded and shapedarticles made therefrom are useful in a variety of applications such ashandles and grips for tools or utensils, as well as weather strip forautomotive and construction uses.

1-11. Cancel.
 12. A foam produced from a composition comprising: A)polyolefin thermoplastic resin, B) olefinic elastomer which is at leastpartially cured, C) an acrylic-modified polytetrafuoroethylene polymer,and D) optional additives.
 13. A process for foaming an olefinicthermoplastic elastomer comprising incorporating into said thermoplasticelastomer, prior to foaming, an effective amount of an acrylic-modifiedpolytetrafluoroethylene.
 14. The process of claim 13 wherein saidacrylic-modified polytetrafluoroethylene is incorporated in the range offrom about 0.1 to about 4 weight percent, based on the total weight ofthe thermoplastic elastomer composition.
 15. The process of claim 13using a blowing agent for said foaming wherein said blowing agent isselected from the group consisting of water, steam, a water-generatingmaterial, or mixtures thereof.
 16. A foamed article prepared using theprocess of claim
 13. 17. The composition of claim 12 wherein saidacrylic-modified polytetrafluoroethylene polymer is present in the rangeof about 0.1 to about 4 weight percent, based on the total composition.18. The composition of claim 17 wherein said acrylic-modifiedpolytetrafluoroethylene polymer comprises polytetrafluoroethylene whichhas been modified with an alkyl (meth)acrylate having from 5 to 30carbon atoms.
 19. The composition of claim 12 wherein said polyolefinthermoplastic resin is selected from the group consisting ofpolyethylene, polypropylene and mixtures thereof.
 20. The composition ofclaim 12 wherein said olefinic elastomer is selected from the groupconsisting of ethylene-propylene copolymer rubber,ethylene-propylene-diene terpolymer rubber, and mixtures thereof. 21.The composition of claim 1 wherein said olefinic elastomer has beendynamically vulcanized so that less than 5 weight percent of theelastomer is extractable.
 22. A molded or shaped article comprising thefoam according to claim
 12. 23. The article of claim 22 where used as ahandle or grip for tools or utensils.
 24. The article of claim 22 whereused as weather strip for automotive and construction.